Organic esters, represented by the generic formula, RCO.sub.2 R' wherein R and R' are independently selected from an organic functionality, are utilized in numerous applications. Recent data indicate that more than 600 esters are currently sold in the United States and more than 100 esters are available in medium and bulk lots. On the basis of bulk production, poly(ethylene terephthalate) is prepared in greatest quantity and is used in manufacturing polyester fibers and thermosetting fibers. Poly(ethylene terephthalate) is typically prepared by reacting terephthalic acid and ethylene glycol or by transesterification of dimethyl terephthalate with ethylene glycol.
Numerous catalytic processes are known for producing organic esters including reacting an alcohol and either an acid anhydride, an acid chloride, an amide or a nitrile in the presence of a suitable catalyst under reaction conditions sufficient to form the desired product. For example, organic esters can be prepared by reacting a carboxylic acid and an alcohol in the presence of catalysts such as strong mineral acids, tin salts, organo-titanates, silica gel and cation-exchange resins. Unfortunately, these reactions proceed via a reversible equilibrium and can be driven toward completion only by removing the desired ester product or water.
Highly volatile organic esters such as methyl formate, methyl acetate and ethyl formate, typically have lower boiling points than their corresponding alcohols and can be readily removed from the product mixture by conventional methods. However, water cannot be easily separated by simple distillation from aliphatic alcohols and esters of medium volatility because the product mixture forms an azeotrope. Consequently, processes for making such esters are highly energy intensive because the water/ester azeotrope must be broken in order to recover the desired ester.
U.S. Pat. No. 3,510,511 discloses a process for preparing an ester by reacting a carboxylic acid and an alkyl ether. The process comprises continuously introducing an ether of an alkanol and a carboxylic acid simultaneously in a proportion to two moles acid to one mole ether, into a boiling mixture initially consisting of sulfuric acid and the ether in a proportion of 0.7 to 1.3 mol of sulfuric acid per mol of ether; continuously extracting the vapors evolved from the reaction mixture and isolating the ether product from water via fractional distillation.
Derevitskaya and coworkers, Tetrahedron Letters, 49 (1970) 4269 disclose a process for preparing alkyl esters by reacting an alkyl tert-butyl ether and a carboxylic acid in the presence of a catalytic amount of a proton-donating agent such as sulfuric acid or para-toluenesulfonic acid. The reaction is represented by the formula: EQU R--O--tert--C.sub.4 H.sub.9 +R'--CO.sub.2 H.fwdarw.R'--CO.sub.2 R+CH.sub.2 .dbd.C(CH.sub.3).sub.2 +H.sub.2 O.
The driving force of the above-mentioned reaction results from evolution of isobutylene. The desired organic ester is separated from the reaction mixture by diluting the mixture with diethyl ether, washing with aqueous sodium hydrogen carbonate and water and drying over sodium sulfate or magnesium sulfate. Following removal of diethyl ether, the residue is distilled to yield the desired ester.
Considerable research is being conducted in order to develop a process for preparing organic esters which eliminates the shortcomings of the above-mentioned prior art processes. Those skilled in the art of producing organic esters are particularly interested in commercial processes wherein water is not produced in appreciable amounts during the reaction.